1. Field of the Invention
The invention relates to a valve for controlling fluids and more particularly to such a valve especially useful in a fuel injection system.
2. Description of the Prior Art
From European Patent Disclosure EP-0 477 401 A1, a valve is known in which an actuating piston of a valve member is disposed displaceably in a stepped bore of the valve housing, in a part of the stepped bore that has a small diameter. A larger piston movable by a piezoelectric actuator is disposed in a part of the stepped bore having a larger diameter. Between the two pistons, a hydraulic pressure chamber filled with a pressure medium is embodied, so that a hydraulic boost of a motion of the piezoelectric actuator occurs. That is, when the large piston is moved a certain distance by the piezoelectric actuator, the actuating piston of the valve member executes a stroke lengthened by the boosting ratio of the piston diameter, since the piston of the piezoelectric actuator has a larger surface area than the actuating piston of the valve member.
In such valves, 3/2-way control valves are also used, as disclosed for instance in U.S. Pat. No. 5,738,075. Such 3/2-way control valves can be embodied as seat-slide or seat-seat valves. Here the pressure is carried from a rail, for instance, by means of a controllable piston via a pressure chamber and a booster piston to a control chamber, from which the fuel is injected into the combustion chamber. In the nonactuated state, the pressure chamber is connected to a leak fuel line, so that leak fuel pressure prevails in the pressure chamber. However, upon a switchover of the 3/2-way control valve, regardless of the type of valve (seat-slide or seat-seat valve), a communication briefly occurs between the high-pressure region and the leak fuel region. The result is a so-called diversion surge into the leak fuel line. This hinders the relief of the control chamber of the valve until the valve has sealed off the opening to the high-pressure region.
The valve for controlling fluids according to the invention has the advantage that a diversion surge to the leak fuel line of this kind does not occur. It is prevented by disposing a slide element upstream of the seat of the control valve. The slide element is disposed in the opening direction immediately before the piston region with which the piston takes its seat on the valve seat. Depending on the stroke path of the slide element, this makes a delayed opening of the passage between the inflow line and the pressure chamber, but enables an immediate closure of the passage between the inflow line and the pressure chamber. Thus at any instant upon switchover of the control valve, it can be assured that there will be no communication between the inflow line and the leak fuel line. Because of the provision of the slide element, the stroke path of the piston for opening the communication between the inflow line and the pressure chamber is at least equal to but advantageously greater than a stroke path for closing or opening a passage between the pressure chamber of the control valve and the leak fuel line. As a result, the diversion surge into the leak fuel line can advantageously be prevented. Also advantageously, upon switchover of the control valve, the relief of the control chamber is no longer hindered, since because of the slide element, sealing off from the high-pressure region occurs immediately.
In an especially advantageous way, the slide element of the piston is embodied cylindrically. As a result, on the one hand simple production of the slide element can be achieved, and on the other, this makes an overall simple design of the control valve possible.
Advantageously, the piston of the control valve is embodied in two parts. As a result, a seat-slide valve with a pressure chamber disposed between the two parts can be realized. Especially advantageously, the control valve is embodied as a force-compensating valve. The guide diameter at the valve seat then is equivalent to the diameter of the slide element.
Also advantageously, the piston of the stepped bore is embodied in one piece. As a result, a 3/2-way seat-slide valve can be used as an outlet valve.
Preferably, the valve for controlling fluids is embodied as a 3/2-way seat-slide valve or as a 3/2-way seat-seat valve.
Overall, with the valve of the invention, an unambiguous function of a control valve without a diversion surge from the high-pressure inlet region into the leak fuel line can thus be achieved. By varying the stroke lengths or stroke paths of the slide element at the valve seat or the stroke length for closing/opening the communication with the leak fuel line, the overlap at the leak fuel slide element can be optimized.
Depending on the intended application, an optimal slide ratio can be formed of the various stroke lengths. The overlap should be selected such that no significant valve delays occur, yet it is still assured that no diversion surge into the leak fuel line will occur.